Coherent light generators – Particular temperature control – Heat sink
Reexamination Certificate
2000-06-29
2002-09-10
Davie, James (Department: 2828)
Coherent light generators
Particular temperature control
Heat sink
C372S029011, C372S050121
Reexamination Certificate
active
06449296
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a light source used in optical communications, optical transfer technology, and optical information storage technology.
Semiconductor lasers are widely use nowadays as light sources infields such as optical communications, optical data transfer, and optical information storage, because of the coherency of the light radiated therefrom, the possibility of high-speed operation, or their extremely small size.
A semiconductor laser is mounted on a metal component such as a lead frame or metal block, for various reasons such as to ensure a thermal path way because optical output varies subtly with temperature change during the emission of light that has been stimulated by the injection of a current from an external source. However, to mitigate differences in the coefficients of thermal expansion of the metal and the semiconductor material of the semiconductor laser, the laser is first mounted on a mounting body called a “submount” made of a material such as Si or AlN, before being mounted on the metal component.
A semiconductor laser comprises a resonator which has a pair of reflective mirrors and a medium with amplification ratio of at least
1
therebetween. An edge-emitting type of semiconductor laser has become more popular because cleaved facet planes of the crystal can be utilized for the reflective mirrors of the resonator and because the distance through the amplification medium can be easily increased.
On the other hand, it is possible to create a highly reflective mirror using a multi-layer structure of semiconductors and dielectric materials, or the like, and thus implement a surface-emitting laser that emits light in the normal-vector direction of the substrate. However, the technology required for this implementation is not yet sufficient and there are still technical problems to be solved. For example, the surface-emitting laser is still only at the research stage for some materials that is necessary to realize the required wavelength of emitted light. This is why most of the semiconductor laser light sources used in manufactured products are of the edge-emitting type.
When an edge-emitting semiconductor laser is mounted on a mounting body, however, problems such as those described below occur. An active region of the semiconductor laser can be used as a wave-guide structure having an extremely small cross-section, to increase the amplification efficiency and thus prevent losses due to the leakage of light from the amplification region, but this will cause diffraction of the light beam emitted from the end facet so that it expands.
In general, since it is possible to use crystal growth techniques or the like to form a thin region of wavelength order of magnitude in the direction perpendicular to the element substrate, light can be confined into a region of wavelength order of magnitude. In a direction parallel to the substrate, on the other hand, it is difficult to confine light within the wavelength order of magnitude because the confining region is formed of a planar structure, and the confining is also done within a region that is broader than the wavelength, even though that prevents any rise in the element resistance.
For that reason, the angle of diffraction diverges much more in the perpendicular orientation than in the parallel orientation. The angle of divergences of a light beam in the perpendicular orientation and the horizontal orientation become different.
For example, in the perpendicular orientation the divergence angle is on the order of 30 degrees that subtends 1/e
2
of the optical intensity on the optical axis. In contrast to this, in the horizontal orientation the divergence angle is approximately 10 degrees that subtends 1/e
2
of the optical intensity on the optical axis.
When a device is mounted on a flat mounting body, a part of the light beam comes into contact with the mounting surface in the vicinity of the element (such as within a distance of 200 microns when the light-emitting portion has a height of 100 microns from the mounting surface). Thus, the part of the light beam is obstructed, due to reflection, scattering and/or absorption at the mounting surface.
This will have an adverse effect during the connection of an optical fiber to an optical pickup that uses this light beam. It is therefore necessary to use some skill when mounting the device, in order to prevent kicking in the vicinity of the peripheral edge of the mounting body. This makes it necessary to limit the positional relationship between the semiconductor laser and the mounting surface, reducing the degree of freedom of installation.
One method of solving the above problems is proposed in Japanese Patent Application Laid-Open No. H05-315700. That is, a semiconductor laser element is mounted in a recess portion formed in a silicon substrate that acts as a mounting body, and a light beam that has been reflected by a wall surface of the recess portion is extracted as information.
With this configuration, the output light is reflected upward in the vicinity of the semiconductor laser element, in other words, before it diverges greatly, making it possible to extract an output beam that substantially retains its original shape, with little interference from the mounting surface and without having to consider any particular positional relationship with the mounting surface.
The optical output of a semiconductor laser varies subtly with changes in the ambient temperature. For that reason, both the semiconductor laser and the mounting substrate are mounted together on an element that enables temperature control, such as a Peltier cooler.
However, the mounting substrate and the mounting body substrate have a certain thermal capacity despite their small dimensions so, if accurate optical output control is required, the actual output beam is monitored and feed back control is imposed on the driving current circuitry. This is called automatic power control (APC).
In an edge-emitting semiconductor laser, two end facets formed at cleavage planes or the like are used as mirrors, but the output beam that is emitted is symmetrical in the forward-backward direction, provided that the reflectivity of the end facets is not particularly limited. It is possible to configure this APC by using a light-detecting element to monitor the light that is output from the rear, but this reduces the utilization efficiency of the light because the monitored light does not contribute to the signal.
For that reason, a system that requires a higher output or a higher efficiency could employ a method of increasing the utilization efficiency of the light by simply using a dielectric multi-layer film to increase the reflectivity of the end facet at the rear. In such a case, the light emission from the rear that can be used for monitoring is reduced, worsening the S/N ratio and impeding accurate APC.
This makes it necessary to monitor part of the light emitted from the forward end (the signal light), and this control method is called front-monitored APC (hereinafter abbreviated to FAPC). It should be noted, however, that if part of the beam is divided up for the purpose of FAPC, the output beam shape can change, in a similar manner to that of the obstruction of the beam described previously.
To prevent this problem, the previously mentioned Japanese Patent Application Laid-Open No. H05-315700 proposes the structure described below. The configuration, as shown in
FIG. 9
, is such that a mirror
925
formed on the mounting body
902
is made of a semi-transparent film and a n-type diffusion region
924
is formed behind the mirror
925
by a method such as thermal diffusion. The p-n junction that is formed around the border of the n-type diffusion region
924
acts as a photodiode element to detect the emitted light from the laser diode
901
.
This structure ensures that the shape of the light beam that is reflected by the semi-transparent film
925
is emitted substantially unchanged, although the light beam has a reduced intensity. In addition, the format
Furuyama Hideto
Hamasaki Hiroshi
LandOfFree
Semiconductor laser device does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Semiconductor laser device, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Semiconductor laser device will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2876675